If you specify a protocol in the build settings for a WSA application, your project cannot be built anymore as the Package.appxmanifest is improperly generated

---
Generated:
<Extensions>
<Extension Category="windows.protocol">
<Protocol Name="unity-azme" />
</Extension>
</Extensions>

---
Expected:
<Extensions>
<uap:Extension Category="windows.protocol">
<uap:Protocol Name="unity-azme" />
</uap:Extension>
</Extensions>

Fixed on 5.4.
Reproducible on 5.3.

Repro steps:
1. Create an Empty Project.
2. Add a protocol name in the WSA settings.
3. Build the project for WSA Universal 10.
4. Open the project in Visual Studio.
5. Try to open the Package.AppxManifest. Error appears "Unable to Load Manifest Designer".

Workaround: Edit protocol tags in Package.AppxManifest.